The present invention is directed to a method of improving the corrosion-resistant properties of metal-filled organic coatings on a metal substrate and to the product thereof. More particularly, this invention is directed to enhancing the galvanic properties of zinc-rich organic coatings on steel substrates. Improving the corrosion resistant properties of a product as a means to extend the life thereof has been a concern of mankind for many, many years. The concept of providing a metal or protective layer over a substrate, to electrochemically protect the substrate is the oasis of one of the oldest processes for protection against corrosion. Electrogalvanizing and hot-dip galvanizing, i.e. coating steel with zinc, are two methods which have been practiced for years to protect steel.
A more recent development is the use of metal-filled organic resin systems. One such system is taught in U.S. Pat. No. 3,849,141 to Palm, et al., and to other systems referred to therein. Briefly, a pre-paint coating composition is applied to a cleaned steel base followed by a top coating of a zinc-filled organic resin.
The role of the metal-filled layer, is to allow electric resistance spot-welding, and to provide protection to the underlying steel as a barrier between the elements of the atmosphere and the steel, and through cathodic protection. Zinc, insofar as it relates to steel, is an ideal filler metal to satisfy this role described above. Accordingly, the further description herein shall be directed to the use of zinc and its function with regard to this invention.
Protection through the imposition of a barrier layer is simply to isolate the underlying steel substrate from the corrosive actions of the environment. There are many materials which can at least provide some initial protection to steel by such mode of protection. However barrier layer protection has its limits due to weathering, erosion or especially damage to the barrier layer as a result of forming operation, welding, impact or scraping. Accordingly, another important mode of protection for steel is what has become known as sacrificial or galvanic protection.
Galvanic protection of steel is an electrochemical process involving the steel substrate, the zinc layer, and the atmosphere. Through contact of the three elements of the system, i.e. steel, zinc, and electrolyte, an electric potential is established. As a result of the more noble position of iron over zinc in the electrochemical series of metals, the zinc is sacrificed thereby retaining the integrity of the iron or steel. The formulas for the electrochemical action taking place are as follows:
o - Dissolution of zinc EQU Zn.fwdarw.Zn.sup.+2 +2e.sup.-
o - Reduction Reactions on Steel EQU 1/2O.sub.2 +H.sub.2 O+2e.sup.31 .fwdarw.2OH.sup.-
in neutral or alkaline solutions, or EQU 2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O;
in acid solutions.
A key element in this entire reaction process is electrical contact between the zinc and the steel so that an electric current can flow. It will be seen from the specifications which follow that through the application of an electric discharge treatment, a zinc-rich organic coating can be transformed from a mere barrier coating to a combined galvanic/barrier coating which affords cathodic protection to bare steel exposed at cut edges and at locations of damage to the coating.